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Forum: Hadronic Processes
Re: Question Fission in the 1-40 MeV range
Date: 09 Oct, 2013
From: Jose Manuel Quesada <Jose Manuel Quesada>

Hello Kaj,

           unfortunately there is no simple solution to meet your needs. In principle it is possible to switch off the fission just by doing a little of hard coding. In Geant4, above 20 MeV,  fission is treated at the model level (i.e. it has not an associated cross section). I'll refer to the G4 native de_excitation models: in the evaporation subfolder of de_excitation hadronic model you can set fission probability to 0 in G4CompetitiveFission class (take into account that fission, as well as photon evaporation, is treated as a competitive channel with nucleon and light cluster - d,t,3He & alphas- emission). In this way you just switch off fission and you can introduce your new fission process (and associated cross section). But you must keep in mind that  the "inelastic" cross sections  for hadronic interactions now in Geant4 are actually "reaction" cross sections, which include all non-elastic channels, being fission just one among them. Therefore, you  would needed to create  your new fission process (with its associated cross section) competing at the process level with the "bare" inelastic process (namely, without fission); obviously,  for avoiding overcounting, you must  (simulataneously to  introducing your own new fission process)   subtract its cross section from the present inclusive "inelastic" one  to obtain the new "bare" inelastic process.

 On the other hand, ABLA  fission/evaporation model, originally developed at GSI, does a nice job describing fission and IMF production in spallation reactions. It is used as the final de_excitation stage by INCL cascade model in Geant4. Your energy range of interest is much lower, but maybe Davide Mancusi (developer end maintainer of  those models in Geant4 ) can give you some hints of their performance in your energy range.  

Finally; I guess that discontinuity at 20 MeV will be present unless you don't use HP neutron physics.

 Best regards

                     Josť Manuel 

.

On Tue, 08 Oct 2013 13:30:40 GMT, KJan wrote:

> Hello Geant4 community,
> 
> in our group we are looking for a way of simulating an experimental
> setup with neutrons up to 40 MeV. We are especially interested in
> fission. The HP models do a great job up to 20 MeV but the cascade
> models (Bertini, Binary cascade) in the 20-40 MeV range are not very
> good. Especially there's a clear discontinuity at 20 MeV.
> 
> The HP models are quite complex and I have not been able to find
> documentation for the data format. It looks to me that it will be
> difficult just adding more cross section data to G4NDL for the needed
> elements thus extending the range that way.
> 
> So what can we do? Is it possible to shut off the fission channel in the
> cascade models and implement our own fission model in the 20-40 MeV
> range? Have I missed some other model which does a better job on fission
> above 20 MeV? Or is there a better model taking care of inelastic
> scattering above 20 MeV but not doing fission (thus disturbing any by us
> implemented fission model > 20 MeV)?
> 
> Any ideas and suggestions are welcome, Kaj
> 

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